A. Why One Side Coating? PA0 B. General Characteristics of Hot-dip Galvanizing PA0 C. Galvanizing Modifications PA0 D. Attempts at One-Side Coating
This invention is directed to apparatus and a method for the continuous production of one-side coating of a metallic strand. More particularly, though not limiting in its application, the preferred invention is directed to one-side galvanizing of a ferrous base strip.
Armed with the knowledge of years of experience on the manufacture and use of two-side metallic coated steels, such as galvanized, aluminized, and several proprietary aluminum-zinc coatings, and recent and costly attempts to produce a suitable metallic coating on only one side of a ferrous strand, one may logically ask why sacrifice corrosion protection of one side, especially at a premium cost? The answer is simple--one must respond and be prepared to meet the demands of the consumer/customer. Specifically the automotive industry, spurred by the public for more corrosion resistant car bodies, demanded it. The automakers wanted steels for auto bodies that could offer corrosion resistance to the underside while providing the outside with a suitable surface having the uniformity and paintability of cold-rolled steep strip. It has long been recognized that even special treated galvanized (two-side coating) did not offer a suitable surface for painting. Hence, the answer was one-side galvanized steel strip. Though the answer was simple, the means to achieve the end were not.
Inasmuch as the preferred invention herein deals with continuous hot-dip galvanizing, or at least certain aspects thereof, it may be helpful to review some general characteristics of a galvanizing process.
One of the first commercial practices still followed today is taught in U.S. Pat. No. 2,110,893, to Sendzimir. Sendzimir discloses a continuous galvanizing method whereby a ferrous strand, such as steel strip, is passed through a high temperature oxidizing furnace to produce a thin film of oxide coating on the steel strip. The strip is then passed through a second furnace containing a reducing atmosphere which causes a reduction of the oxide coating on the surface of the steel strip and the formation of a tightly adherent impurity-free iron layer on the steel strip. While the strip remains in such reducing atmosphere, the steel strip is immediately immersed in a molten zinc bath maintained at a temperature of about 850.degree. F. (456.degree. C.). The strip is then cooled in air, or by accelerated means, resulting in a bright spangled surface.
There is a modified gas cleaning process that is practiced today for the production of galvanized steel. Such a process uses a Selas-type direct-fired furnace and avoids the initial high temperature oxidizing treatment of the Sendzimir process.
A third practice which has also gained acceptance for galvanizing steel strip is described in U.S. Pat. Nos. 2,824,020 to Cook et al, and in 2,940,870 to Baldwin. The practice described by such patents includes the step of applying a flux to the strip to be galvanized. The flux acts as a cleaning agent producing an oxide-free strip surface, which readily coats with the molten zinc bath.
From each of such processes there is produced a galvanized coating which is ductile. However, such coating has a major drawback in that its spangled surface is too rough to permit a smooth paint finish. Further, when painting is desired costly surface preparation is generally required.
To produce a non-spangled surface more adaptable to painting, without further treatment of the surface, a high temperature post heat treatment was introduced for the coated strip. This process is known as galvannealing.
The preparatory steps and the coating step are identical to the spangled or unalloyed version. After the coating immersion step the coated strips follow different processing sequences. In U.S. Pat. No. 3,322,558 to Turner, a process is taught wherein the coated strip, as it leaves the galvanizing bath, is passed upwardly between rows of open burners. These burners are mounted in such a way as to minimize the effect of emissivity of the sheet and maximize heating of the strip by convection heat. This uniform heating of the strip at temperatures from 900.degree. F. to 1200.degree. F. (483.degree. to 649.degree. C.) results in a uniform dull finish where the coating surface is fully alloyed.
In the above galvannealing process, the zinc coated strip is heated to above the melting temperature of zinc, i.e. about 790.degree. F. (421.degree. C.), to accelerate the reaction of zinc with the coating base iron. This results in the growth of the intermetallic layer from the iron base to the surface. Thus, a characteristic of galvannealed strip is a fully alloyed coating and the absence of spangles. While the introduction of such galvannealing treatment appeared to provide an answer to improvements in the paintability of galvanized steel, a loss in coating ductility was found.
To overcome the inherent problems with unalloyed and fully alloyed galvanized strip, while retaining the desirable properties thereof, an intermediate product was developed. Such product, characterized as a partially alloyed galvanized coating in U.S. Pat. No. 4,059,711, is achieved by a method which carefully controls the alloying activity of zinc with the ferrous base in a continuous galvanizing operation.
Metal coating practices to coat one side only of a continuous strip in a hot-dip coating operation have been known for many years. However, none have led to a large scale commercial practice. An early attempt is disclosed in U.S. Pat. No. 1,252,363, to Roberts. Such patent discloses apparatus and a method for coating tin on one side of a moving strip. In the process the strip is guided through a molten bath of tin where clamping collars along the strip edges prevent the molten tin from flowing over the strip edges. Such collars effect a fluid-tight joint at the strip edges below the surface of the bath.
Later developments approached the coating on one-side only of a continuous strip in a different manner. Such developments were characterized by the step of treating the scheduled uncoated side in such a manner that such side would not be wetted by the molten coating metal. U.S. Pat. No. 3,383,250, to Pierson et al, accomplished this by treating the surface to be free of coating metal with an air blast or oxygen, thereby producing on such side an oxide layer. A variety of further attempts, U.S. Pat. No. 3,149,987 to Crandall being exemplary of such attempts, treated the surface to be free of coating metal with a masking agent, such as sodium bentonite.
A very recent attempt to coat only one side of a moving strip is the meniscus process taught in U.S. Pat. No. 4,082,868, to Schnedler, et al. In the Schnedler process the strip surface to be coated is caused by travel sufficiently close to the molten coating metal bath surface that the surface tension and wetting characteristics of the coating metal will permit the formation of a meniscus which will continuously contact and coat the one side of the traveling strip.
The present invention, to be described hereinafter, avoids the mechanical clamps of Roberts, the surface protection and removal thereof of the masking practices, and the delicate controls of the meniscus process.